The invention relates to a discharge lamp comprising a discharge vessel in which are arranged two electrodes, between which a discharge current flows in the operating condition of the lamp, which vessel is enclosed by an outer envelope provided with a neck which is adjoined by a sleeve of a lamp cap.
Lamps of the kind mentioned in the opening paragraph are frequently used nowadays both in public illumination and in illumination in residental rooms. These lamps have the great advantage of a comparatively high luminous flux with small dimensions as compared with incandescent lamps and low-pressure discharge lamps. The filling can consist of a combination of one or more metal vapours and one or more rare gases, or of one or more metal halides, mercury and rare gas.
Such lamps, and especially high-pressure discharge lamps, are generally operated in combination with a stabilization ballast. The value of the impedance of the ballast should be chosen to match the discharge current through the lamp in the operating condition. Consequently it is not possible to operate a lamp of one kind in an existing equipment designed for operating a different kind of discharge lamp, without making modifications to the existing equipment. This is a disadvantage because of the progressive increase in luminous efficacy in new types of high-pressure discharge lamps, and the attempts to obtain a further saving in energy.